putIfAbsent method
V putIfAbsent(This operation is not supported by an unmodifiable map.
Source
V putIfAbsent(K key, V ifAbsent()) { throw new UnsupportedError("Cannot modify unmodifiable map"); }
© 2012 the Dart project authors
Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dartlang.org/stable/1.24.3/dart-collection/UnmodifiableMapBase/putIfAbsent.html